Should you wear a tracker?

Sleep trackers are better than they were and worse than they claim. Whether you should wear one depends less on the device and more on how you’re likely to react to it.

What they measure well

Time in bed, roughly when you fell asleep, and your heart rate. Movement. Overnight temperature, on newer devices. These are genuinely useful, and the trend over weeks is more reliable than any single night.

What they estimate

Sleep stages. A wrist device inferring deep versus REM sleep from movement and heart rate is making an educated guess, and when compared against proper laboratory measurement, those guesses are mediocre - decent at distinguishing asleep from awake, considerably worse at the stage breakdown.

So the number that gets the most attention in the app is the one you should trust least.

The failure mode

There’s a recognised pattern where tracking makes sleep worse: you sleep adequately, the app reports a poor score, you spend the day believing you slept badly, and you go to bed anxious about the next score. The anxiety then produces the poor sleep the app was wrongly reporting.

If you recognise yourself in that, take the device off. The data is not worth what it’s costing you.

How to use one well

Look at weekly averages, not nightly scores. Use it to answer specific questions - does alcohol affect me, is the room too warm, does my Sunday lie-in hurt Monday - and then stop looking. A tracker is a measuring tool for occasional experiments, not a report card.

The alternative

How you feel at eleven in the morning tells you most of what a tracker does. If you’re alert without caffeine at that hour, you’re sleeping enough, whatever the app says.